1. Automotive Cooling Technology Options
1.1 Typical Driving Scenario
In order to estimate the fuel savings afforded by an adsorption air conditioner, it is necessary to define the basis of the estimation, which will be the typical commute to and from work. In the USA, driving mileage is distributed 38% commuting, 35% for running family businesses, and 27% for social, recreational, and religious activities. The average yearly mileage has increased from 16,539 km in 1990 to 18,870 km in 1999 [1], so a round estimate of 19,312 km.yr−1 (12,000 miles.yr−1) is assumed for 2006. Therefore, the average round trip commute is 30.6 km=(19,312 km.yr−1×38% commuting÷240 work days).
Assuming the typical commute is 50% city driving and 50% highway driving, and that the average city street speed is 56 km.hr−1 (35 miles.hr−1) and the average highway speed is 97 km.hr−1 (60 miles.hr−1), the typical commuter spends 16 minutes on city streets and 10 minutes on freeways each day. Also, the typical commuter spends 62 hours per year idling (at stop signs, traffic signals, freeway on ramps, and in traffic jams) in rush hour traffic [2]. This equates to another 15 minutes per day. Thus the total daily commute time is 16+10+15=41 minutes, ≈20 minutes to work and ≈20 minutes returning home.
1.2. Automotive Cooling Requirements and Current Mechanical Compression Technology
The cabin of a car parked in the open for a couple of hours on a sunny and warm, not necessarily hot, day will get very hot because the large area windows admit sunlight but trap infrared radiation emanating from the interior (i.e., the greenhouse effect). This is called “hot soaking.” Interior temperature can easily reach 60° C. on a warm (25° C.) day, and exceed 70° C. on a truly hot (35° C.) day [3].
FIG. 1 shows the effect of operating the air conditioner on fuel mileage for three classes of automobiles (subcompact, compact, and midsize) during highway cruising, city driving, and idling, and includes the amount of waste heat generated during each scenario. For example, the air conditioner of a midsize car must have about 7 kW capacity to cool the “hot soaked” cabin to a comfortable temperature within 10 minutes after start-up [3, 4]. During the initial 10 minute “surge cooling” interval, the air conditioner runs continuously (i.e., 100% duty cycle), after which it runs intermittently to maintain a comfortable cabin, remaining on about ⅓ of the time (33% duty cycle), providing an average of 2.3 kW cooling. Thus, for the typical 20 minute commute, the average cooling load is 4.7 kW [=(7 kW×10 min.+2.3 kW×10 min.)÷20 min.].
The Coefficient of Performance for Cooling [COPC, a measure of efficiency equal to cooling (kW) divided by work input (kW)] of automotive air conditioners is quite low. Heat pumps commonly exhibit COPC=3.8 or greater for state-of-the-art stationary (e.g., residential) applications with a modest temperature “lift” (Tlift=Tcond−Tevap) of 45° C.−5° C.=40° C. However, COPC drops to about 2.1 for vehicles, because Tlift is increased to about 58° C.−3° C.=55° C. to permit smaller condensers and evaporators, and each ° C. increase in Tlift results in a 2% to 4% decrease in COPC [3.8(1-3% avg.×ΔTlift)=3.8(1−0.03×15° C.)=2.1]. Mechanical compressors are compact and light with Specific Cooling Power, SCP≈1000 W.kg−1 [5].
1.3 Potential Fuel Savings
The fuel savings to be realized from an exhaust powered air conditioner is that due to eliminating the parasitic power consumption of the mechanical compressor. The overwhelming majority (>97%) of light duty vehicles (cars, vans, pick-ups, SUVs) employ spark ignition (Otto cycle), gasoline burning engines, with state-of-the-art thermal efficiency of 30%. The remaining 70% of the heat of combustion is dissipated as heat, about 35% via the radiator and 35% in the exhaust at city and highway speeds where ram air induction effectively cools the radiator [1]. At idle, a larger portion of waste heat (≈⅔=67%) is discarded via the exhaust. Even the minimal 3.5 kW of exhaust heat from an idling subcompact (see FIG. 1) should be enough to power a regenerative (heat recycling) adsorption heat pump providing 1.7 kW cooling (at 33% duty cycle) needed to maintain cabin comfort after initial surge cooling.
The average parasitic power drain by the compressor of a midsize car during the typical commute is 2.3 kW, and is obtained by dividing the average 4.7 kW cooling load by COPC=2.1, then adding a conservative 2% for belt friction. This average of 2.3 kW is 9.3% of the 25.0 kW needed to propel a midsize car in highway cruising and 14.8% of the 15.6 kW required for city driving (FIG. 1). The idling engine of a midsize car requires about 3 kW to overcome internal friction. Add another 0.7 kW for back EMF from the alternator (at 40 A×14V÷80% efficiency) with A/C off and 1.5 kW for resistance from the torque converter for a total of 5.2 kW. Thus, the compressor comprises an extra 44% load (=2.3 kW÷5.2 kW) on an idling engine.
Therefore, the average midsize car with a mechanical compressor consumes 16% more fuel when the air conditioner is used during commuting.
                                                                                                              16                    ⁢                                                                                  ⁢                    min                    ×                                          (                                              15.6                        +                        2.3                                            )                                        ⁢                                                                                  ⁢                    kW                                    +                                                                                                                          10                    ⁢                                                                                  ⁢                    min                    ×                                          (                                              25.0                        +                        2.3                                            )                                        ⁢                                                                                  ⁢                    kW                                    +                                      14                    ⁢                                                                                  ⁢                    min                    ×                                          (                                              5.2                        +                        2.3                                            )                                        ⁢                                                                                  ⁢                    kW                                                                                                                                                                (                                          16                      ⁢                                                                                          ⁢                      min                      ×                      15.6                      ⁢                                                                                          ⁢                      kW                                        )                                    +                                                                                                                          (                                          10                      ⁢                                                                                          ⁢                      min                      ×                      25.0                      ⁢                                                                                          ⁢                      kW                                        )                                    +                                      (                                          14                      ⁢                                                                                          ⁢                      min                      ×                      5.2                      ⁢                                                                                          ⁢                      kW                                        )                                                                                      =                  1.16          =                      16            ⁢            %                                              (        1        )            A heat pump powered by presently wasted exhaust would eliminate the 16% additional power needed to operate the mechanical compressor during the typical commute. Viewed differently, an exhaust powered heat pump will reduce fuel consumption an average of [(116%−100%)÷116%]≈14% during air conditioner operation. Substituting values for compact and subcompact cars from FIG. 1 into the above equation yields fuel savings of 17% and 18%, respectively.
Assuming the cooling season averages four months for the USA, the annual fuel savings is ˜5% for small to midsize vehicles. Europe as a whole is the second largest automobile market, after the USA, but has significantly lesser need for automotive cooling, although the percentage of new vehicles equipped with air conditioning is rapidly increasing nonetheless. For sunnier Southern European locations bordering on the Mediterranean (Iberia, southern France, Italy, Greece, and the Balkans), savings similar those for the Southern USA “Sunbelt” would be expected, significantly greater than the nationwide average of ˜5%. Greater than 5% savings would be expected in the large market (rivaling Europe) comprised of equatorial South America (principally Brazil, followed by Northern Argentina then Venezuela and Columbia), Australia, South Africa, Saharan Africa (principally Egypt), the Near East, Middle East, and Southeast Asia (principally Singapore, Taiwan, and South Korea). But fuel economy may not matter much in the Middle East. Although China and India have the #1 and #2 populations (⅓ of the world's) and are rapidly developing, consuming ever greater amounts of oil, reliable inexpensive transportation is still the order of the day, so air conditioning automobiles is not yet a significant consideration. Central Africa has a huge population and large cooling requirement, but it is a small automobile market with basic transportation as the goal. Japan, although a major automobile market (fourth), has minimal need for automotive cooling.
1.3 Utilizing Waste Heat
There are three potential uses for waste heat in a vehicle: (a) cabin heating, (b) cabin cooling, and (c) electricity generation, the latter of which could be used for heating and cooling. Heating is already performed efficiently, compactly, and economically by routing engine coolant through a small finned tube heat exchanger (HEX) in the cabin air duct. The only drawback is the long delay (5+ min.) during frigid weather between engine start-up and effective cabin heating and defrosting.
Alternators are typically rated at 105 A×14 V=1.5 kW, which equates to a mechanical load on the engine of about 2.0 kW, assuming η=75%. Average alternator load is about half rated output, since most driving is done in daylight with the lights off, the cabin fan is usually on low or medium instead of high, and the wipers are seldom used. The fuel pump runs constantly, but the thermostatically controlled radiator fan often shuts off at city cruise speed. So the power drain by an alternator at 50% (50 A) of rated output is about 2.0 kW×50%=1.0 kW.
A thermoelectric generator directly powered by exhaust heat could conceivably replace the alternator, and power motors connected to the water pump, power steering pump, and compressor. The average power drawn by the compressor during a typical 20 minute commute is 1.6 to 2.3 kW (FIG. 1), equal or greater than for all other ancillary equipment combined (≈1.0 kW by the alternator plus ≈0.7 kW by the water and power steering pumps). So, eliminating the compressor provides the greatest boost to efficiency.
Using a thermoelectric generator to power a motor driving a compressor is only one method of eliminating its parasitic power drain. There are a number of thermal effect devices which can convert wasted exhaust heat directly into cabin cooling without having to go through the intermediate step of producing electricity with the attendant losses in efficiency. Alternative cooling technologies are reviewed next to determine the best option.
1.5 Alternative Cooling Technologies
1.5.1 Stirling Cycle Cooling
Recent improvements in efficiency of reversed Stirling cycle systems (achieving COPC=3.0) still do not approach the efficiency of the best reversed Rankine cycle designs (COPC≧5). Also, a reversed Stirling cycle heat pump requires work input. But it cannot be belt driven by the engine if the goal is to eliminate the parasitic power loss associated with cabin cooling. So the heat pump must be driven by a motor that is powered by a thermoelectric generator. This combination of three components would add considerable mass. Each energy transformation from heat to electricity to mechanical work to heat (cooling) would incur loss in efficiency.
A state-of-the-art design [6] is reported to have SCP=12 W.kg−1, with COPC=3.0, and Tlift=20° C., about one-third of the required Tlift≈55° C. Increasing Tlift to the required range will markedly reduce COPC and SCP. But, even ignoring this degradation of performance, a reversed Stirling cycle heat pump capable of delivering 7 kW of cooling would have a mass of 580 kg [=7000 W=12 W.kg−1], not accounting for the considerable mass of the thermo-electric generator.
1.5.2 Absorption (Liquid-Vapor) Cooling
The major difference between the liquid-vapor absorption chiller and the mechanical-vapor-compression heat pump is the primary form of energy used to power the cycle. The vapor compressor is replaced by a liquid pump, which requires a fraction of the power (≈4%) to pump the much denser liquid solution of refrigerant and absorbent to high pressure. A burner or solar collector or low quality heat from a power plant or industrial process heats the “generator” causing refrigerant to desorb from the absorbent. The most common refrigerant-absorbent pairs are ammonia-water (NH3—H2O) and water-lithium bromide (H2O—LiBr). Absorption systems attain COPC≦0.65 to 0.70 for “single effect” heating cycles which do not recover and reuse heat after it passes through the system. “Double effect” heating or “heat recovery” (i.e., recycling of heat) yields up to COPC=1.2, but such devices are bulkier, more complex, and costlier.
Boatto et al. [4] constructed an automotive absorption system. They had difficulties in designing major components to meet geometrical and functional specifications for integration in a car. High system mass was also a problem. Separation of the refrigerant from the absorbent in the “generator” was strongly affected by acceleration and vibration. They suggested that the best refrigerant-absorbent pair was H2O—LiBr, but cautioned that corrosion by the hot brine was a challenging problem. Boatto et al. [4] concluded that preheating the brine with engine coolant entailed too many complications, and so chose to remain with a system employing single effect heating that used only exhaust heat and yielded COPC≈0.5.
1.5.3 Absorption (Solid-Vapor) Cooling
Solid-vapor adsorption is similar to liquid-vapor absorption, except that the refrigerant is adsorbed onto a solid desiccant (freeze dried) rather than absorbed into a liquid (dissolved). The adsorption cycle is illustrated in FIG. 2 and proceeds as follows:                a. At state 1, a cool canister, or adsorber, contains adsorbent saturated with a large fraction of refrigerant at slightly below Pevap. The cool adsorber is heated and desorbs refrigerant vapor isosterically (i.e., at constant total mass in the adsorber), thereby pressurizing it to state 2, slightly above Pcond. At this point vapor starts being forced out the hot adsorber, through a one-way “check” valve to the condenser.        b. Isobaric heating desorbs more refrigerant, forcing it out the adsorber and into the condenser until state 3 is attained, whereat the adsorber is nearly devoid of refrigerant.        c. The hot adsorber is then cooled isosterically (at constant total mass) causing adsorption and depressurization, until the pressure drops below Pevap (state 4) opening another check valve to allow vapor to enter the adsorber from the evaporator.        d. Isobaric cooling to state 1 saturates the adsorbent, completing the cycle.        
Thus, the mechanical compressor can be replaced with one or more adsorbers. Cyclically and asynchronously heating and cooling two or more adsorbers results in continuous cooling. Solid-vapor heat pumps require a low quality heat source at typically 150 to 250° C. Catalyzed automobile exhaust is usually at least 400° C., even at idle.
Prototype adsorption (solid-vapor) systems with innovations for recycling heat (up to 75% to date) have achieved COPC=1.2 [7, 8]. Also, adsorption systems can be designed to be unaffected by acceleration and vibration, do not use highly corrosive brine, and can be smaller for a given capacity than absorption systems. An SCP of 220 W.kg−1 of adsorbent has been demonstrated [9] and SCP=590 W.kg−1 adsorbent has been predicted [10].
Three adsorbent-refrigerant pairs have received the most attention to date: zeolite (a class of highly nano-scopically porous, alkali-alumino-silicate minerals with cage-like crystalline lattices)-water, activated carbon-ammonia, and silica gel (SiO2)-methanol (CH3OH).
1.5.4 Thermoelectric Cooling (Peltier Devices)
Although Peltier coolers can exhibit up to η=45% [11], they tend to have very low SCP of 10-25 W.kg−1 [12], as compared with SCP=1000 W.kg−1 for a mechanical compressor [5]. Also, an automotive Peltier device would need an exhaust powered, thermoelectric generator with efficiency η≦5% [13] and extremely low SCP=0.25 W.kg−1 [14]. The cost is $4 to $5 per watt [11].
Thus, although simple in concept, a thermoelectric cooling system would exhibit a mere η=45%×5%=2.2%, far too low to be powered by engine exhaust. It would provide no more than 0.44 kW cooling for a compact car, as compared with 6 kW needed for surge cooling (FIG. 1). The Peltier device would have a mass of 240 kg (=6 kW÷25 W.kg−1), and the thermoelectric generator, assuming a very optimistic SCP=10 W.kg−1, would have a mass of 1300 kg [=6 kW÷10 W.kg−1)÷45%].
1.5.5 Selection of the Most Promising Alternative Cooling Technology
Of the four alternative technologies reviewed above, adsorption (solid-vapor) cooling is the most promising. The other three alternate cooling technologies are either unfeasible or not as promising as solid-vapor adsorption cooling for reasons described above.
2. Literature Review: State of the Art in Adsorption Heat Pumps
2.1 Simple Cycle Adsorption Heat Pumps Not Utilizing Heat Recycling
Solid-vapor adsorption heat pumps were used in domestic refrigerators and railroad cars in the 1920's and 1930's [15]. The COPC for built and tested simple cycle (i.e., “single effect” heating with no recycling of heat) adsorption heat pumps is 0.3 to 0.4 [9]. This is primarily due to the fact that heat rejected from the adsorbent during the cooling phase was simply discarded. A second reason for low COPC and SCP is that much of the mass (the pressure vessel and its internal heat exchanger) is non-adsorbing, or so-called “dead,” mass that is unavoidably heated and cooled with the adsorbent but contributes nothing to the compression effect.
2.2 Recycling Heat to Increase COPC 
COPC can be increased by recycling heat that is necessarily rejected from the adsorbent bed being cooled by transferring it to the adsorbent bed being heated, thereby reducing the required external heat input (“make up” heat). A heat transfer fluid (HTF: oil or glycol-water solution) is used to exchange heat between beds. The effectiveness of heat recycling depends upon how the heat is transferred from the bed being cooled to the bed being heated, which is bounded by two extremes: (a) uniform temperature heat recovery or “double effect” heating, and (b) “thermal wave” regeneration described below.
2.2.1 Uniform Temperature Heat Recovery or “Double Effect” Heating
Uniform temperature heat recovery or double effect heating (FIG. 3) can reduce required “make-up” heat by about 40% in a two-bed device, boosting COPC from 0.3 to 0.4 for single effect (no recycling of heat) adsorption devices to 0.5 to 0.65 [0.3÷(1−0.4)=0.5; and 0.4÷(1−0.4)≈0.65]. Once the beds reach equal temperature, double effect heating is no longer possible (FIG. 3). Thus, the theoretical limit of heat recovery for a two-bed device is 50%, but the aforementioned 40% is the practical limit [9].
2.2.2 “Thermal Wave” Regeneration
“Thermal wave” regeneration results from employing moving temperature gradients or “thermal waves” that traverse the adsorbent beds to heat and cool them (FIG. 4) and was first suggested by Tchernev and Emerson [7]. Thermal wave regeneration is more efficient than uniform temperature heat recovery for a given number of beds, since heat is transferred across a smaller temperature difference, creating less entropy. Tchernev et al. [7, 8] demonstrated 75% thermal wave regeneration, elevating COPC to about 1.2 [0.3÷(1−0.75)=1.2]. The theoretical maximum efficiency for thermal wave regeneration is 100% for an infinitesimal ΔT between HTF and adsorbent; however, the practical limit has been estimated at 85% [8].
2.3 Synopsis of State-of-the-Art in Adsorption Heat Pumps
The current state-of-the-art in adsorption heat pumps has been reviewed. Research groups in the United States, Italy, France, China, and Japan have concentrated their efforts [17-29] on devising improvements to the all-critical adsorbers, with the primary goal of improving efficiency (COPC), which requires increasing the percentage of recycled heat. Several investigations, e.g., [17, 19-24, 26, 28], agree in identifying the two most important parameters that must be maximized in order to increase COPC: (1) the ratio of adsorbent (“live”) mass to non-adsorbent (inert or “dead”) thermal mass Cads/Cinert, and (2) the NTU of the heat exchanger. Since they have been working to maximize COPC for stationary applications, little effort has been directed toward increasing SCP, which is at least as important as COPC for transportation applications.
According to Lambert and Jones [16], some designs suffer from low thermal mass ratio Cads/Cinert, the first of the two critical governing parameters identified above. And most also suffer from low NTU, the second critical governing parameter, because they do not distribute heat effectively due to small contact area Acontact between the HEX and a given volume of adsorbent ads, such as the concentric tube configurations in FIGS. 5 and 6. Thermal resistance due to small Acontact is exacerbated by the typically poor junction conductance kjunc between the metallic HEX and the nonmetallic adsorbent. FIGS. 7 and 8 show two configurations with substantially greater Acontact, a shell-&-tube type (FIG. 7) and a spiral tape type (FIG. 8) devised by Wang et al. [25]. But these latter two types provide much greater Acontact at the expense of markedly lower Cads/Cinert. The one exception is the flat pipe serpentine HEX winding between consolidated adsorbent tiles designed by Tchernev et al. [7, 8], as shown in FIG. 9. However, this design posed insurmountable problems in manufacturability, reliability, and expense, owing to its delicate configuration and sub-atmospheric pressure that allowed for air leaks into the system.
Another limitation of previous designs is that none embody a satisfactory method for increasing the poor thermal conductivity of adsorbents kads while retaining sufficient permeability to refrigerant vapor. Consolidating adsorbents into bricks increased kads and marginally increased junction conductance hjunc but decreased vapor permeability by 3 to 4 orders of magnitude [7, 8, 18-20, 22, 23]. Binders used in consolidation occlude pores. Some designs use a coiled tubing HEX inside beds of packed spheres [17], resulting in a very low effective kads. None of the studies consider settling of the adsorbent particles, which may cause adsorbent to lose contact with the heat exchanger. Performance parameters for several investigations are compiled in FIG. 10.